Tikalon Blog by Dev Gualtieri

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Flash Bainite

June 29, 2011

What could be scientifically exciting about steel , especially to someone with a physics background? I once remarked that a physicist is the type of materials scientist who tries to find steel in the periodic table . A physicist is also the type of computer scientist who writes a database in Fortran . I'm guilty on both counts, although in my case, I used Visual Basic instead of Fortran. I even wrote a web browser in Visual Basic. It operated on local server files and used my own hypertext markup language before "http" and ubiquitous use of the Internet splashed onto the scene.

Graduate students discover that much of their education is tied to the special interests of their professors . When you've worked on a topic for many years, sometimes to the exclusion of all others, it starts to seem like the most important thing in the world. So, when a professor teaches a course, he's likely to return to his pet topic, again and again. That's how I became an expert in Ostwald ripening , as I discussed in a previous article (Ostwald Ripening, March 23, 2007).

For the same reason, I got to know quite a bit about the Martensitic transformation , a diffusionless transformation that's important in the manufacture of steel. This type of transformation from one crystal structure to another involves just the slightest rearrangement of atoms , usually less than the interatomic spacing , and that's why it's termed diffusionless.

The Martensitic transformation is responsible for the shape-memory effect in certain alloys, notably Ni ₅₅ Ti ₄₅. If you gently bend a piece of this Martensitic material at room temperature , it will retain the bent shape. Heating the piece above a particular temperature will transform the Martensite to austenite , the piece will revert to its original shape, and it will keep that shape even when cooled again to room temperature, forming Martensite. This (austenite <--> Martensite) transformation is reversible for NiTi, but it is not reversible in steel. Steel is not a shape-memory alloy.

In steelmaking, Martensite is a body-centered tetragonal crystal phase of iron and carbon . It's produced by cooling from austenite, a face-centered cubic phase. The presence of Martensite makes steel harder . Martensite is not an equilibrium phase of the iron-carbon system, so it can't be found in the equilibrium phase diagram . This phase transition has a very small activation energy , since it involves such a slight movement of carbon atoms, so it can even occur at cryogenic temperatures in some materials. Mechanical stress can cause the transformation, also.

Bainite is another non-equilibrium microstructure formed from austenite that produces a hardened steel. It forms upon rapid cooling through a temperatures range of about 300 - 500 °C, depending on the presence of alloying elements . Bainite was discovered in the 1920s, but it's seen a renewed interest in a material called Flash Bainite, invented by a small Detroit company, SFP Works, LLC .[1-5]

The patent pending process[2] for Flash Bainite is claimed to produce steel that's seven percent stronger than Martensitic steel. It involves an extremely rapid heating and cooling cycle; thus, the "flash" appellation. The process has been described in a paper in the journal, Materials Science and Technology,[3] and a schematic diagram of the process appears below.[2]

Figure caption

The Flash Bainite^TM process, as described in its US patent application, "Method and Apparatus for Micro-Treating Iron-Based Alloy, and the Material Resulting Therefrom," by Gary M. Cola, Jeff W. Ziolkowski and Todd C. Ziolkowski.

The roll steel product is passed quickly through a heating section (17) and a cooling bath (23).

(Figure 2 of US Patent Application 11/718,969, Publication No. US 2007/0261770 A1, November 16, 2005).[2]

An example of the Flash Bainite process, as described in this paper, subjects a sheet of AISI-8620 steel to a rapid heating and cooling cycle of less than ten seconds duration. The hot zone temperature is about 1100°C , and the cooling is by a water quench . The resultant product has a high ultimate tensile strength (1694 MPa ) and a high elongation (7·1%). This is a 7% higher strength and 30% greater elongation than high strength Martensitic steel (1585 MPa, 5·1%). Microscopy showed a distribution of bainitic and Martensite microstructures with carbides . As claimed on the company web site, their Flash 4130 product exceeds the strength-to-weight ratio of the titanium alloy, Ti-64, at just 10% the cost.

Of course, the thermodynamics of the process requires a thin working material, but there are abundant applications in sheet and tubing products. The process is claimed to be environmentally friendly, also. Only a kilowatt /kilogram of energy is needed to heat the feedstock, and the cooling is by a water quench, not with oil or molten salt .[4]

Gary Cola, the inventor of Flash Bainite is a self-trained metallurgist , so he tapped into the scientific expertise of Ohio State University to validate his process.[5] He enlisted the aid of Suresh Babu , an associate professor of materials science and engineering at Ohio State, who admits that steelmaking is a "mature technology" with very few surprises.[5] Babu is also Director of the co-located National Science Foundation Center for Integrative Materials Joining for Energy Applications .

The material properties tested true, and the Ohio State team investigated the mechanism. A typical slow cooling will produce Martensite from an austenitic steel, but the flash process is so rapid that the carbide phase does not dissolve completely during heating, and you end up with a complex microstructure of bainite, Martensite and carbides after cooling.

All this reminds me of a Design of Experiments course I took many years ago. In a designed experiment, you write down all the possible variables of a process, and then run experiments at the limits of each variable. If cooling rate and heating rate are two variables, you would do four experiments with (cooling rate, heating rate) set at (high, high), (low, low), (low, high) and (high, low). Flash Bainite would fall into the (high, high) class, but the trick is knowing what's really high, and what's really low. A "skilled" metallurgist would likely never venture into the rates as extreme as those for Flash Bainite.

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